1. Field of the Invention
The present invention relates to a drive controller for a lens apparatus, which controls driving lens units in order to prevent an image plane variation caused by changing magnification.
2. Description of the Related Art
Hitherto, in a consumer-oriented zoom lens apparatus, a movement speed of a magnification changing lens unit (zoom speed) during a zoom operation is limited to a speed capable of tracking an auto focus (AF) operation of a focus lens unit and tracking data (data indicating a position of the focus lens for maintaining a position of an image plane substantially steady when the magnification changing lens unit moves). In other words, a maximum movement speed of the magnification changing lens unit depends on a controllable movement speed of the focus lens.
Therefore, an actual zoom speed according to the same command value of zoom speed is high in a wide side but becomes lower toward a telephoto (tele) side. Thus, zooming cannot be quickly performed in the tele side in many cases.
U.S. Pat. No. 5,438,190 discloses a method for preventing an image plane variation caused by a movement of a magnification changing lens unit, the variation being prevented by a focus lens. In this method, a standard speed depending on positions of the magnification changing lens unit and the focus lens is corrected according to a depth of field and a focus state while the magnification changing lens unit moves, and the focus lens is driven based on the corrected standard speed.
On the other hand, in a business-use zoom lens apparatus for broadcasting or the like, zooming is manually performed in general, unlike in a consumer-oriented apparatus. By manually performing a zoom operation, extremely high-speed zooming is realized. In other words, the zoom speed cannot be limited in a range in which a control of preventing an image plane variation by the focus lens unit can follow the zoom speed. As a result, an out-of-focus state is likely to occur at high-speed zooming.
For example, when a position of an image plane is maintained by continuing an AF operation of searching for a position where an AF evaluation value extracted from a video signal is higher while driving the focus lens unit with micro amplitude during zooming, a favorable image-plane maintaining performance can be ensured at low-speed zooming. However, at high-speed zooming, the image-plane maintaining performance degrades because some time is required for generating an AF evaluation value and for search drive and determination of direction of the focus lens unit.
When a plurality of pieces of tracking data are prepared, a piece of tracking data of a direction where the evaluation value is higher is searched for by moving a second lens unit to a minimum object distance (MOD) side and an infinity (INF) side in order to select a piece of tracking data to be used at zooming. Also, in order to more effectively suppress an image plane variation during zooming, an operation of searching for the tracking data is repeatedly performed during zooming. In this case, however, an image-plane maintaining performance degrades because time is required for searching for tracking data, as in the AF operation.